Standby circuit using a two filament incandescent lamp to maintain approximately thesame light output



STANDBY CIRCUIT USING A TWO FILAMENT INCANDESCENT LAMP TO MAINTAINAPPROXIMATELY THE SAME LIGHT'OUTPUT Filed NOV. 4, 1964 I May 9, 1967 w.sM|TH 3,319,115

INVENTOR W/LZ/flM 7. 577/ 779 M/% if ATTORNEYS United States Patent3,319,115 STANDBY ClRCUIT USING A TWO FILAMENT 1N- CANDESCENT LAMP TOMAINTAIN APPROXI- MATELY THE SAME LIGHT OUTPUT William T. Smith, 419Stratford Court, San Antonio, Tex. 78223 Filed Nov. 4, 1964, Ser. No.409,030 3 Claims. (Cl. 315-66) The invention described herein may bemanufactured and used by or for the United States Government forgovernmental purposes without the payment to me of any royalty thereon.

This invention relates to an incandescent electric lamp of highreliability, and, more particularly, to an incandescant lamp having twolight emitting filaments that will provide approximately the samecontinuing illumination after one filament has become inoperative suchas through burn-out or mechanical breakage.

The life of electric illuminating lamps, also commonly referred to aslight bulbs, pilot lights, indicatorlights,

signal lights, and the various combinations of such words, is quiteunpredictable. This is particularly so if the bulb is used for blinkingor intermittent service; and even more so if subjected to variousamounts of mechanical shock, coupled perhaps also with a fluctuatingsupply voltage. The inadvisability is well understood of having anindicating light fail at a crucial time, such as when the system bearingthe light is experiencing mechanical shock or vibration and theindication is necessary in order to know the corrective action to beundertaken.

Various ways have been utilized to overcome the aforementioneddifficulty. The most common and most obvious is to use a systemincorporating multiple incandescent lamps with two or more lamps toilluminate a common object, or perform a common indication. This methodis very wasteful of space and costly due to the multiplicity ofcomponents. Furthermore, after a lamp failure the quantity ofillumination is materially reduced.

It is, therefore, an object of the present invention to provide anincandescent lamp that will have long life.

Another object of the present invention is to provide an incandescentlamp that will indicate when an appreciable portion of its useful lifehas passed.

Another object of the present invention is to provide an electric lampwith two light emitting filaments that will maintain essentially thesame amount of illumination after one filament has ceased to emit light.

Another object is to provide an electric lamp that will beinterchangeable mechanically with conventional bulbs, requiring noexternal controls or adjustments, and yet provide greatly improvedreliability.

Additional objects and features of the herein disclosed. invention willbecome apparent to those skilled in the art after examining thefollowing detailed description and drawings in which:

FIG. 1 is a drawing of an embodiment of the invention as it may beincorporated in a small bayonet base pilot light.

FIG. 2 is a drawing showing an arrangement of the filiments and controlresistor that provides equal heat transfer from each filament to thecontrol resistor.

FIG. 3 is a schematic drawing showing the voltage and currentdistribution in .a specific embodiment of the invention.

FIG. 4 is a schematic drawing of the same embodiment of FIG. 3 showingthe voltage and current distribution after one filament has becomeinoperative.

Referring to FIG. 1, a bayonet base bulb of the pilot lamp type is shownincorporating the advantages of the present invention. Base 8 is of theconventional single tip contact bayonet type. Electrical connections aremade to an external source of electrical power through the insulated tipand the shell of the base. Glass envelope 7 is cemented to the base 8with conventional adhesive 10. Light emitting filaments 1 and 2 areessentially identical, conventional, electric lamp filaments. They areconnected in parallel by the electric current carrying supports 6 and 4.The temperature sensitive current control resistive element 3 is seriesconnected with the parallel filaments through the common conductivesupport 4. Conductor 5 connects the control element to the external tipcontact 11. The supporting and conducting stiff wires 4, 5 and 6 areheld in place by the molded or fused glass stem member 9, which is alsoan atmospheric seal maintaining the atmospheric integrity of lighttransmitting glass envelope 7. The sealed interior is evacuated orfilled with an inert gas to prolong the life of the filaments bypreventing their oxidation, as is done in conventional illuminatinglamps having but one filament.

FIG. 2 shows an embodiment of the invention in which the current controlelement 22 is parallel, adjacent and equally spaced apart from thefilaments 20 and 21. Due to the equal amount of heat received by controlresistor 22 from the filaments 20 and 21 this arrangement is generallyto be slightly preferred to the vertical plane arrangement of FIG. 1. Asin FIG. 1, the light transmitting envelope 23 seals the inert atmospherecontained within the envelope from the outside atmosphere. Theconductive support members 26, 27 and 28 perform the same function asthe similar members of FIG. 1 even though in this embodiment they are ofslightly different shape. Solid glass stem member 24 is fused about theconductors and maintains them in their proper position. The screw typebase has a conductive outer member 25 electrically connected tosupporting and conducting member 27. Insulated center tip contact 29 iselectrically connected with member 26.

The light transmitting glass envelope 7 of FIG. 1 and 23 of FIG. 2 maybe either clear or frosted. However, if frosted, it is desirable thatthe frosting be not so dense as to prevent the observance of the lack oflight source from one of the filaments. Thus, if a lamp is replaced soonafter one filament burns out, essentially uninterrupted illumination maybe obtained by utilizing lamps made according to the teaching of thisin.- vention. The physical spacings one from another of the filamentsand the control element are not critical; however, they should beseparated such that no physical inis to be avoided due to theprobability of the are formed at the time of breaking of one filamentdamaging the re maining filament. From the foregoing, it is seen that itwould be desirable to operate the lamp of FIG. 1 with its major axis andthe filaments in a horizontal plane. The lamp of FIG. 2. is ideallysuited for operation in a vertical manner. If conventional filamentsupporting wires are used. at the approximate midpoint of each filament20 and 21, the bulb may be used in any position and the probability ofthe shorting of remaining members by abroken filament member would bevery low. The current control resistive element may be covered with orencased in an insulative coating to further preclude the possibility ofmalfunction caused by the loose end of a broken filament making anelectrical contact with the control element. Likewise, to furtherpreclude the occurrence of an electrical mal-function,the supportingmembers for the filaments and the control resistor may be coated with aninsulating material.

In order to understand the operation and theory of the current controlresistor, it is first desirable to examine the following table, whichpresents some of the parameters of a typical tungstenfilament whenoperated at normal and at reduced voltages as .a percentage of the valueat normal rated voltage.

4 FIG. 4 shows the redistribution of the voltages and currents afterfilament 42 fails. With 27 volts across the single filament,approximately 90 percent of normal illumination is obtained the same asshown in FIG. 3.

Percentage Percentage Percentage Percentage Percentage Condition 01 RateRated Rated Rated Rated Voltage Resistance Lumens Watts Amperes Thevalues shown are approximate and they are de- Here, control resistor '80has changed sufiiciently to propendent upon the size of the lamp (i.e.the designed watt input or the designed candle power output), thedesigned voltage input, the designed efficiency, and the nominaldesigned life of the lamp. Examine condition 6. The voltage has beenreduced to 80 percent of the rated value. The resistance of the filamenthas gone down ten percent due to its lower temperature, but the lightoutput in lumens has dropped to 40 vpercent of that value of light given01f at rated voltage. From closer examination of the table, it can 'beseen that a relatively small change in voltage will produce a relativelylarge change in the amount of illumination output. It is to be observedin condition 6 that electrical power input to the filament has onlydecreased to 70 percent of the normal value. Thus, the combined radiatedenergy (heat and light) has not decreased as much as the light outputhas decreased indicating that the emitted heat-to-light ratio has goneup.

A very specific example and embodiment of this invention will be setforth as an example. Due to the multiple interactions of the parametersof this lamp, it is advisable that the final determination of thecontrol resistor in a specific embodiment be determined empirically.

A standard 28 volt aircraft indicator light has the followingcharacteristics for normal (100 percent) light output:

Volts 28 Amperes .1 Watts input 2.8 Ohms resistance 280 If the voltageinput to the lamp is reduced to the value of 27 volts, then to 23 volts,the following conditions exist:

Watts Ohms Percentage Volts Amperes Input Resistance of NormalIllumination It is now apparent that with two standard 28 volt .1 amperefilaments operating at 23 volts, approximately the same amount ofillumination is obtained as from one filament operating at 27 volts;i.e., approximately 90 percent of the full amount illumination from onefilament. The power in watts dissipated in radiant heat and light energyfor the one filament at 27 volts is 2.66 watts; for the two filaments at23 volts it is 4.14 watts.

Referirng to FIGS. 3 and 4, the example under discussion in diagrammed.In FIG. 3, control resistor 80 produces a 5 volt drop when bothfilaments 32 and 33 are operating with .18 amperes of current flowingfrom 28 volt source or energy 31. This places 23 volts across theparallel filaments 32 and 33 and 90 percent of the illumination of onefilament at full rated power is obtained. Neglecting any slight changein the resistance of one filament due to the heat received from theother, the value of control resistor should be approximately 28 ohms.

duce only one volt of vot-lage drop. That is, it should now haveapproximately 10.4 ohms resistance.

Two methods of obtaining the desired resistance variation in currentcontrol resistor have been found to be feasible. The first is by using acurrent sensitive resistor that is effectively out of the heat field ofthe light and heat emitting filaments (or filament), that has aresistance of 28 ohms when .18 amperes of current is flowing through itcausing it to dissipate .9 watts of energy, and a resistance change toapproximately 10.4 ohms with .096 ampere flowing and a resultingdissipation of approximately .096 watt. Thus, a resistor that has apositive resistance current characteristic of approximately 1.5 to 1.0would give the desired control. Practically, in most instances, a linearcontrol having a positive resistancecurrent characteristic ratio from 1to 1, to 2 to 1 is suitable.

The second method of obtaining the same control characteristics is byusing a current control resistor that is essentially temperaturesensitive instead of current sensitive. It has been shown that with bothfilaments operating at approximately 23 volts, approximately 4.14 wattsof energy, mainly in the form of heat, is radiated. Then when only onefilament remains in operation approximately 2, 66 watts should bedissipated and radiated, mainly as heat. Thus, by controlling the areaand surface condition of the control resistor, the amount of heatreceived and reradiated by it can be controlled with the resultingcontrol of its temperature. Assuming, for illustration, the temperaturesensitive current control resistor is sufficiently higher in temperaturewhen the two filaments are radiating 4.14 watts that when one filamentis radiating 2.66 watts to produce a resistance change of approximately1.5 to 1. The current control resistance thus provides for a greatercurrent flow through a single filament, after the failure at the otherfilament, than the amount of current flow previously provided througheach filament when both were operating. The desired control is thenobtained; in this instance, without any inherent resistance change inthe control resistor caused by the current flowing through it.

In practice, both of these methods may be used simultaneously andcooperatively since they both tend to reinforce each other. That is, theresistance is a positive direct function of both the current flowingthrough it causing internal heating and its environmental temperature.Positive temperature coefficient control resistor 80 may be fabricatedfrom conventional resistance wire such as all-oys of nickel and chromiumor it may be fabricated from tungsten similarly to another filament. Asemiconductor material of the thermistor class that has a positivetemperature coefficient may also be used as the current controlresistor.

In the illustrative embodiment shown in FIGS. 3 and 4, the metalliccurrent control resistor 80 has a cold resistance of approximately 10ohms. When both filaments are active, its resistance due to its elevatedtemperature is approximately 28 ohms. With only the one filament activeits temperature is considerably lower and its resistance isapproximately 10.5 ohms.

Generally, the degree of control of the illumination is not extremelycritical and it is only required that the illumination stayapproximately the same after one filament has failed.

Other modifications and changes in the number and arrangement of thecomponents of the incandescent electric lamp set forth herein may bemade by those skilled in the art without departing from the nature ofthis invention, within the scope of the appended claims.

I claim:

1. The improvement in an incandescent electric lamp, heated toincandescence by a flow of electric current, hav ing a lighttransmitting enclosure with exterior supporting and electricalcontacting means afiixed on the exterior of said lamp, the saidimprovement comprising: two similar filaments for radiating light andheat energy; support means for supporting said filaments insubstantially parallel physical relationship within said enclosure;means electrically connecting said filaments in electrical parallelrelationship; electrical resistive means responsive to the said heatenergy having a resistive characteristic approximately proportionate toits temperature for increasing the current flowing in one of the saidtwo filaments after failure of the other; means for positioning the saidresistive means within the said enclosure in adjacent, parallel, equallyspaced apart, heat receiving relationship to the said filaments; meansconnecting the said electrical resistive means and the parallelconnected filaments in series; and electrical connecting meansconnecting the said series connected parallel filaments and resistivemeans to the said electrical contacting means.

2. The improvement of claim 1 wherein the said electrical resistivemeans is tungsten wire.

3. The improvement of claim 1 wherein the said electrical resistivemeans is resistance wire of an alloy comprising nickel and chromium.

References Cited by the Examiner UNITED STATES PATENTS 996,489 6/1911Key 315-192 X 1,156,064 10/ 1915 Deschere 315-192 X 1,654,295 12/ 1927Loewe 315-107 1,800,903 4/1931 Ramsey 315-93 X 2,152,228 3/1939 Waters315-71 2,161,443 6/1939 Warshawsky 315-68 2,293,045 8/ 1942 Crowell338-20 X 2,675,502 4/1954 McMahan 338-20 X 2,875,377 2/1959 Woo 315-71 X2,904,764 9/1959 Minter 338-20 2,981,699 4/ 1961 Ichikawa 338-20 X3,148,016 9/1964 Fisher 315-93 X FOREIGN PATENTS 5 34,641 3/1'922France.

JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner.

1. THE IMPROVEMENT IN AN INCANDESCENT ELECTRIC LAMP, HEATED TOINCANDESCENCE BY A FLOW OF ELECTRIC CURRENT, HAVING A LIGHT TRANSMITTINGENCLOSURE WITH EXTERIOR SUPPORTING AND ELECTRICAL CONTACTING MEANSAFFIXED ON THE EXTERIOR OF SAID LAMP, THE SAID IMPROVEMENT COMPRISING:TWO SIMILAR FILAMENTS FOR RADIATING LIGHT AND HEAT ENERGY; SUPPORT MEANSFOR SUPPORTING SAID FILAMENTS IN SUBSTANTIALLY PARALLEL PHYSICALRELATIONSHIP WITHIN SAID ENCLOSURE; MEANS ELECTRICALLY CONNECTING SAIDFILAMENTS IN ELECTRICAL PARALLEL RELATIONSHIP; ELECTRICAL RESISTIVEMEANS RESPONSIVE TO THE SAID HEAT ENERGY HAVING A RESISTIVECHARACTERISTIC APPROXIMATELY PROPORTIONATE TO ITS TEMPERATURE FORINCREASING THE CURRENT FLOWING IN ONE OF THE SAID TWO FILAMENTS AFTERFAILURE OF THE OTHER; MEANS FOR POSITIONING THE SAID RESISTIVE MEANSWITHIN THE SAID ENCLOSURE IN ADJACENT, PARALLEL, EQUALLY SPACED APART,HEAT RECEIVING RELATIONSHIP TO THE SAID FILAMENTS; MEANS CONNECTING THESAID ELECTRICAL RESISTIVE MEANS AND THE PARALLEL CONNECTED FILAMENTS INSERIES; AND ELECTRICAL CONNECTING MEANS CONNECTING THE SAID SERIESCONNECTED PARALLEL FILAMENTS AND RESISTIVE MEANS TO THE SAID ELECTRICALCONTACTING MEANS.